Impaired nucleocytoplasmic transport (NCT) of protein and RNA through the nuclear pore has recently emerged as a central mechanism in neurodegeneration. Indeed, we have recently shown that mutant huntingtin markedly exacerbates aging-related alterations in nuclear integrity and disruption of NCT, and defects in nuclear pore-mediated transport have also been uncovered in C9ORF72-related amyotrophic lateral sclerosis (ALS) and frontotemporal dementia (FTD). Here, we demonstrate that expression of mutant FUS, a protein linked to ALS and FTD, leads to a loss of nuclear pore integrity, altered nuclear envelope morphology and other phenotypes related to disrupted NCT in multiple, relevant models including isogenic human neurons and a novel FUS knock-in mouse model. In this project, we will further interrogate the relationship between disease-associated FUS and impaired NCT in ALS and FTD using the aforementioned model systems and human CNS tissues (Aim 1). To elucidate the mechanism by which expression of mutant FUS induces NCT-related phenotypes, we will further probe the interactions between FUS and nuclear pore proteins, and test the hypothesis that abnormal phase transitions involving mutant FUS and nuclear pore proteins contribute to disrupted NCT (Aim 2). We also posit that impaired NCT induces toxicity via the mislocalization of transcripts and proteins, a notion that will be tested by using RNA-sequencing and proteomics to compare gene expression after cellular fractionation in mutant FUS versus control neurons, before and after manifestation of NCT-related phenotypes (Aim 3). Finally, we will test the therapeutic potential of targeting nuclear export in FUS human neurons and mice using strategies that have already been shown to be both safe and effective across multiple human diseases (Aim 4). This synergistic collaboration between two academic groups and an industrial partner (who will provide the therapeutic compound for studies in our laboratories) has the potential to uncover new mechanistic insights to disease and establish the therapeutic potential of targeting nucleocytoplasmic transport in ALS/FTD.